A BASINAL EXPRESSION OF THE LATE SILURIAN (LUDFORDIAN) EXTINCTION EVENT: INTEGRATED SEDIMENTARY, BIOTIC, AND PALEOREDOX DYNAMICS IN A CARBONATE PLATFORM FROM SOUTHERN LAURENTIA

The Silurian was a time interval of environmental upheaval marked by transitions between climate states, extinctions, and major perturbations to the global carbon cycle. The most notable of these was the mid-Ludfordian Lau extinction, which is one of the most severe biotic events in the Silurian and was originally recognized as severe conodont extinction. This extinction event coincides with the Lau positive carbon isotope excursion (CIE). Here we examine a basinal expression of the Lau event interval through paired carbon and sulfur isotope analyses, along with I/Ca ratios, that have been generated from four carbonate sections along the southern Laurentian paleo-margin. The relative age of these sections has been well-constrained by previous conodont biostratigraphy. The geochemical data is complemented by detailed litho- and bio-facies analyses at the macroscopic and microscopic scales for assessment of sedimentary and biological changes. The petrographic analyses also provide context of the diagenetic history for each sample. The I/Ca proxy provides data to infer local redox conditions as low ratios suggest intervals of low dissolved [O₂] or anoxic waters near the formation and preservation of the carbonate mineral, whereas elevated ratios indicate more oxygenated conditions. Positive excursions in the δ³⁴S_pyr records denote a local increase in the burial of pyrite. The δ¹³C and δ³⁴S_CAS records from this region lend evidence in support of global increases in organic carbon and pyrite burial likely due to the expansion of euxinia in late Silurian oceans. Importantly, the integrated results show that geochemical proxies varied in tandem with carbonate microfacies characteristics and fossil assemblages, indicating substantial biogeochemical evolution occurred throughout the studied interval. Furthermore, this local to global research in carbonates supports previous proxy work in shales that record a global increase in the extent of reducing conditions that is stratigraphically coincident with the known Lau extinction and provides a critical driving mechanism for the extinction.